The present invention relates to apparatus for disintegrating extrudates from various types of extrusion granulators in which moistened powder raw materials are forced through die holes to form generally cylindrical extrudates.
Extrusion granulators are widely employed in the pharmaceutical, food, fertilizer, agrochemical, feed stuff, catalyst, ceramics, and like industries for the production of granules generally on the order of from less than one millimeter (1 mm) diameter to several millimeters in diameter. In the typical production of granules by extrusion granulators, powder raw materials are initially moistened by mixing with liquid and/or binding agents. The moistened raw materials are often further treated in a kneading apparatus to obtain improved plasticity and are then extruded through multiple die holes in the extrusion granulator. Each cylindrical extrudate which emerges through the die holes hangs from the die surface due to gravity until its own weight causes it to sever or break. Therefore, the granules obtained in this manner are of random length, and generally have a widely varying aspect ratio (i.e., the ratio of the longitudinal dimension of the extrudate to its transverse or diametric dimension), which can range on the order of from ten to several hundred depending upon the extrudate's strength and plasticity. Moisture in the extrudates is conventionally removed in various types of dryers such as a fluid-bed dryer, a rotary dryer, or a band-type dryer to obtain acceptable final moisture contents.
In many industries, granules with homogeneous length and small aspect ratio are increasingly demanded depending upon the intended application. For example, in the Japanese pharmaceutical industry, variations in granular form should be within a predetermined size limitation, i.e., +10 Mesh=0(%), +12 Mesh .ltorsim.5(%) and -42 Mesh.ltorsim.15(%). To meet these requirements, the dried extrudates are treated in a disintegrator under dry conditions in which large conglomerates of the extrudates, which are often formed in the dryer, are disintegrated into uniformly-sized individual granules. Typically, the disintegrator accomplishes a rough grinding of the extrudate to break and separate individual extrudate strands which may have agglomerated to one another into the form of a block or lump. For example, a disintegrator may be equipped with knife-type cutter blades rotating within a screen or a rotating screen with stationary bars or blades disposed therein. The screen around the cutter elements is of a selected mesh size to classify the crushed extrudates by permitting only smaller granules less than the mesh size of the screen to pass the screen and be discharged. The granules thusly produced may undergo a final sifting by a sieving apparatus, if necessary or desired, in order to remove granules which are too fine and too coarse.
The conventional methods described above for the continuous production of uniformly sized granules with a small aspect ratio, i.e., by sequential extrusion granulation, drying, disintegration, and sieving, have disadvantages and a more improved process is demanded in consideration of various desired parameters. For example, during disintegration of agglomerated extrudates, the extrudates are subjected to impulsive force by the rotating elements and make their way through the screen due to centrifugal force created by the rotating element. In many cases, a considerable amount of dust (often referred to as fines) is generated during the process, resulting in a low product yield.
In the case of sticky materials, the wet extrudates emerging through the die holes in the extrusion granulator tend to stick together, forming noodle-like bunched conglomerates, which raise problems in the drying process. If the extrudates are dried in a band-type or stationary tray-type dryer, these noodle-like conglomerates are left in a conglomerate state, and in case of fluid-bed dryers, the fluidizing gas energy is not normally strong enough to disintegrate the conglomerates. In both cases, homogeneous drying cannot be achieved and product yield is low.
If fine granules, e.g., 0.5 mm or below in diameter, are required, a very large drag force may be generated in the granulator due to friction between the extrudates and the internal walls of the die holes, resulting in excessive heat generation and lower production capacity. This problem can be solved to some extent by improving the plasticity of the raw materials to be extruded by increasing the amount of moisture (usually by liquid addition) and/or binder, but this normally causes sticking problems as described above.
Attempts have been made to use a dry disintegrator of the aforementioned type equipped with a screen for the purpose of disintegrating wet and sticky extrudates before drying is accomplished, but this was found unsatisfactory because of screen clogging and material sticking to the interior of the apparatus.
From a production capacity viewpoint, it is also preferable that a process be continuous instead of a batch-type operation.